Add a true waveform display to the oscilloscope plugin.

* Adds some additional niftyness like a floating popup display that
is implemented in an OSD library for use by other plugins.

* Speed changes are now gradual for both views and follow a curve
derived from some fiddling around to get a nice feel.

* Refined a few behavioral things overall.

It needs a bit of help from a direct PCM channel callback so it may
capture PCM for waveform display. Also need a few other core routines
to help out for the OSD.

Messes with some keymaps. Some targets need keymaps to access the
different views. Some devices can't support the additional view
because it requires a large buffer ( > 1 s) for samples.

If the plugin buffer is small, they can still use the popup display
since the plugin is also much smaller in that case.

Slow speed waveform needs some refining so it draws gradually like
a real oscilloscope but I'll stick with what it is, for the moment.

Change-Id: Ieb5b7922a2238264e9b19a58cb437739194eb036
Reviewed-on: http://gerrit.rockbox.org/245
Reviewed-by: Michael Sevakis <jethead71@rockbox.org>
Tested-by: Michael Sevakis <jethead71@rockbox.org>

apps/plugins/lib/SOURCES

@@ -52,6 +52,7 @@ pluginlib_jpeg_load.c
 #endif
 
 checkbox.c
+osd.c
 picture.c
 xlcd_core.c
 xlcd_draw.c